1. Field of the Invention
The present invention relates to a noise detection device that detects noise occurring in each part of a printed circuit board by using a PLL (Phase Locked Loop).
2. Description of the Related Art
In a circuit used in various electrical and electronic devices, and so forth, noise detection and the like is applied in order to reduce the influence of malfunctions caused by noise interference.
Conventional methods of noise detection include, for example, a method that performs detection by convertinga shift in the signal value of the circuit used in the electrical or electronic device, or the similar, into a digital signal by means of an A/D converter and comparing the voltage value with a predetermined value (Japanese Patent Application Laid-Open No. 2000-258238, for example).
In a case where a voltage shift caused by noise is measured by using an A/D converter, because noise has normally a high frequency, high-speed conversion is required of the A/D converter, which makes the circuit very expensive. Because the circuit of the A/D converter increases in size as the speed of the A/D conversion increases, a large A/D converter cannot be installed in an electrical or electronic device with a limited size of a printed circuit board. As a result, the frequency that can be detected according to the circuit scale of the installed A/D converter is limited and detection of higher frequency noise is problematic. There is a problem that, for detection of high-frequency noise, the circuit has to be large-sized, with the result that electrical or electronic devices, and so forth become inevitably large.
There is also a problem that it is difficult to judge whether the detected noise is fatal for the system.
Generally, whether or not each of the circuit elements constituting the system circuit malfunctions due to noise depends on a variety of factors in addition to the noise voltage level, such as the frequency component with noise and randomness, and so forth. For this reason, judgment of whether or not the system is malfunctioning due to noise through the voltage level of the noise alone would be insufficient.
Further, with regard to the whole system in addition to each of the elements, the extent (criticality) of the effect of noise on the system differs according to a variety of factors such as the size of the noise, the noise frequency, the point at which the noise interference invades the system, and the tolerance of the circuit structure to the noise. A system usually has portions resistant to noise and also portions susceptible to noise, so that the system does not malfunction even when there is a large amount of noise at a portion that is resistant to noise, but the system malfunctions when there is even a small amount of noise at a portion that is susceptible to noise, on the other hand. Thus, as the effect of noise is determined according to variety of conditions, there is the problem that detection of the voltage level of the noise for single constituent circuit element would not suffice detection of the effect of noise on the whole system, thus actually making it difficult to determine whether or not there is a level of risk which will cause the system to malfunction.
Further, there is also the problem that mounting of a detection circuit is expensive and a space for mounting such detection circuit is required.